1. Field of the Invention
The present invention relates to an electrophotographic copier, printer, facsimile apparatus or similar image forming apparatus and more particularly to a method of applying a bias voltage for development and a method of switching it in an image forming apparatus.
2. Discussion of the Background
A color image forming apparatus of the type including two image forming units is conventional. The image forming units are spaced from each other by a preselected distance along the same running surface of an intermediate image transfer belt. Each image forming unit includes a single photoconductive element and a plurality of developing sections each for developing a particular latent image formed on the drum with toner of particular color. This type of image forming apparatus is disclosed in, e.g., Japanese Patent Laid-Open Publication No. 10-177286.
The above conventional color image forming apparatus has some problems left unsolved, as follows. In each image forming unit, a plurality of developing sections each develops a particular latent image formed on a single photoconductive drum with toner of particular color in contact with the drum, as stated above. It is therefore necessary to prevent toner of different colors from being mixed by selectively rendering the toner of different colors inoperative by sophisticated control. Moreover, a particular bias voltage for development is assigned to each of developing rollers included in the developing sections. This scales up a bias power source and therefore increases the overall size of the apparatus.